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Cooperative Photochemical Reaction Mechanism of Femtosecond Laser‐Induced Photocoloration in Spirooxazine Microcrystals
Author(s) -
Suzuki Mototsugu,
Asahi Tsuyoshi,
Masuhara Hiroshi
Publication year - 2005
Publication title -
chemphyschem
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.016
H-Index - 140
eISSN - 1439-7641
pISSN - 1439-4235
DOI - 10.1002/cphc.200500251
Subject(s) - femtosecond , photochromism , excitation , photochemistry , laser , photoexcitation , excited state , chemistry , materials science , molecular physics , optics , atomic physics , physics , electrical engineering , engineering
Microcrystalline powders of spirooxazine and spiropyran compounds do not show photocoloration under steady‐state illumination, whereas they undergo photochromism on intense femtosecond laser‐pulse excitation. We investigated the characteristic mechanism of the crystalline photochromism by studying the photocoloration of spironaphthooxazine (SNO) and its chloro‐substituted derivative (Cl‐SNO) with our femtosecond diffuse‐reflectance spectroscopic system. In particular, femtosecond double‐pulse excitation using 390+780‐nm pulses and 390+390‐nm pulses, with a variable time interval between the two pulses, was applied to reveal an intermediate species involved in the photocoloration. Although 780‐nm excitation of an intermediate produced by 390‐nm excitation did not lead to isomerization, the 390+390‐nm excitation resulted in photocoloration. The yield for SNO decreased on increasing the interval from 40 ps to 5 ns, while that for Cl‐SNO was constant. The photocoloration mechanism in the crystalline phase is considered from the viewpoint of the time‐dependent density of short‐lived transient species, and it is concluded that cooperative interactions of excited states and nonplanar open forms play an important role in femtosecond laser‐induced photochromism in these crystals.